Reversible male sterility is a valuable trait in the production of hybrid seed. Because phytohormones are believed to be involved in the plant male organ development, male sterility can potentially be induced by alteration of endogenous hormone levels or by alteration of responses to hormones. Several attempts have been made to chemically induce male sterility with the application of phytohormones (Moore, Science, 129:1738–1740, 1959; Eeninck et al., Acta Bot. Neerla, 27:199–204, 1978; Sawhney, Can. J. Botany, 61:1258–1265, 1981; Saini et al., Aust. J. Plant Physiol., 9:529–537, 1974; Chandra Sekhar et al., Sex. Plant Reprod., 4:279–283, 1991). Although these exogenous hormone treatments successfully produced male sterile plants, the narrow window of application, the necessity of applying chemicals continuously to produce sterility, lower seed yields on the treated plants after crossing, and occasional lapses in achieving complete sterility make hormonal induction of male sterility impractical for commercial hybrid seed production. The pleiotropic effects of the hormone applications as well as environmental influences on exogenous hormone uptake, translocation and metabolism likely cause these inconsistencies. The discovery of genes involved in hormone pathways and promoters conferring tissue-specific expression permit control of endogenous phytohormone levels in specific tissues, and allow precise, effective induction of male sterility. More importantly, male sterility induced by phytohormone perturbations can theoretically be reversed by application of the appropriate phytohormonal agonists or antagonists. Fertility restoration is critical as it enables inbred male sterile lines to be maintained.
The use of molecular biology to produce male sterility in plants has been described. In a series of patents, Cigan et al. disclose the use of the anther-specific promoter 5126 and variants thereof to control expression of sequences related to pollen formation (U.S. Pat. Nos. 5,689,049; 5,689,051; 5,763,243; 5,792,853; 5,795,753; 5,837,851; and 6,072,102). Albertson et al. (U.S. Pat. No. 5,962,769) describe a method for producing reversible male-sterile plants by introduction of an expression vector that produces avidin. Male sterility is reversed by crossing to a “restorer” line expressing anti-sense avidin or a suitable ribozyme. Alternatively, sterility can be reversed by application of biotin. Baudot et al. (U.S. Pat. No. 6,207,883) disclose the male fertility gene Ms41-A in Arabidopsis and a related maize gene Zm41-A. Baudot et al. further disclose that mutation of the Ms41-A gene resulted in male sterility. Michiels et al. (U.S. Pat. No. 6,025,546) utilized a method of transforming plants with a coregulating gene combined with a male sterility gene to generate a higher frequency of male sterile transgenic plants. Poovaiah et al. (U.S. Pat. No. 6,077,991) disclose the suppression of calcium/calmodulin-dependant protein kinase expression by the use of antisense constructs to induce male-sterility. Scott et al. (U.S. Pat. No. 5,955,653) disclose the use of a tapetum-specific callase gene and its promoter to induce male-sterility. Van Tunen et al. (U.S. Pat. No. 6,005,167) disclose a method for inducing male sterility by the use of recombinant polynucleotides that inhibit the expression of one or more genes involved in the synthesis of chalcone or one of its precursors.
It has been reported that sterility in many male sterile mutants is related to the decline of endogenous gibberellin (GA) levels (Nakajima et al., Plant Cell Physiol., 32:511–513, 1991; Sawhney, Can. J. Botany, 70:701–707, 1992; Sawhney and Shukla, Am. J. Botany, 81:1640–1647, 1994) and that mutant plants could also lead to male sterility (Koornneef et al., Physiol. Plant, 65:33–39, 1985).
The gai gene isolated from Arabidopsis can negatively regulate GA responses (Peng et al., Gene Dev., 11:3194–3205, 1997: WO 97/29123). The gai mutation of Arabidopsis confers a dwarf phenotype and a dramatic reduction in GA responsiveness (WO 97/29123). The gai is a gain-of-function mutation, and the wild-type allele (GAI) is dispensable. GAI encodes a polypeptide (GAI) of 532 amino acid residues. A 17-amino acid domain is missing in the gai mutant polypeptide. GAI contains several motifs characteristic of transcriptional coactivators, suggesting that GAI acts as a transcriptional regulator. The mutant gai appears to be resistant to GA regulation.